Rubber treatment method

ABSTRACT

A process and resulting product is provided in which vulcanized rubber crumb has selected chemical bonds broken and sulphur removal by biotreatment with hydrophobic bacteria, such as mycolic acid containing actinomycete bacteria of ‘mycolata’, without significant degradation of the hydrocarbon polymer. The products obtained from the use of these bacteria may be processed alone or in blends with virgin rubber and revulcanized to yield products of much higher quality than conventional reclaimed rubber materials.

The present invention relates to a method of treating rubber. Inparticular, it relates to a method of treating vulcanised rubber inorder that the rubber may be reprocessed or recycled.

BACKGROUND TO THE INVENTION

The largest single application for rubber is in vehicle tyre. Theprincipal rubbers used are the hydrocarbon polymers Natural Rubber (NR),Styrene Butadiene Rubbers (SBR) and Polybutadiene Rubbers (BR). Duringprocessing the polymer molecules are vulcanised i.e. cross-linked bysulphur atoms. The formation of cross-links enhances the mechanicalproperties of the rubber but renders it unsuitable for easy processing.Throughout this application, vulcanised rubber is taken to mean rubbercross-linked by sulphur.

Waste rubber materials, such as vehicle tyres, present a significantenvironmental problem.

Currently, the European Union scraps a total of nearly 2×10⁶ tonnes oftyres per year of which 23% are retreaded and 46% disposed to landfill.In the UK, it is estimated that approximately 100000 tonnes of tyres aredisposed of each year, mainly in landfill sites. In the USA, 300 milliontyres are disposed to landfill per annum. In New York State alone, 12million tyres are discarded per annum, which represents in excess ofthree million barrels of oil in energy equivalent being discarded.

The United Nations and EU have warned that waste rubber is becoming asignificant environmental problem worldwide. The EC Landfill Directive(1999/31/EC) has advocated the banning of disposal to landfill by 2003for whole tyres and 2006 for shredded tyres.

Currently, alternative means of disposal of waste rubber, in particularwaste tyres, include various recycling methods and burning of e.g. incement kilns. However, burning of rubber materials such as tyres canproduce significant pollutants including dioxins.

Life cycle analysis shows that only a small fraction of the energy usedin manufacturing tyres is recovered on combustion.

Conventional recycling methods include mechanical, thermo-mechanical,cryomechanical, microwave and ultrasonic methods. Chemical recyclingmethods include reclamation using organic disulphides, mercaptans andinorganic compounds. However, rubber produced using these methods haspoor mechanical properties. There is also pyrolysis of waste rubber tooils and carbon black. Alternative methods of recycling includeemploying reclaimed rubber that is blended with Low Density Polyethylene(LDPE) to produce mixed polymer elastomeric material. However, rubberproduced using these methods has poor mechanical properties.

Rubber crumb is also used in roads, playparks, running tracks andequestrian surfaces. However none of these applications come close tousing the large quantities of waste rubber available.

A recent biotechnological approach to recycling rubber involveselastomer recycling to the viscoelastic state by removal of sulphur bythe thermophilic bacterium Sulfolobus. However, the use of a thermophileat temperatures in the region of 70° carries a large energy cost penaltyand generates highly acidic environments with production of sulphuricacid which can lead to reprocessing problems. DE04042009 describes thesurface treatment of rubber crumb by suspensions of chemolithotrophicSulphur-oxidising bacteria allowing revulcanisation to take place whenmixed with new (virgin) rubber stock and devulcanisation of comminutedrubber scrap by similar suspensions of chemolithotrophs to produceviscoelastic rubber and sulphuric acid. However, the presence ofsulphuric acid in the reaction mixture would be detrimental torevulcanisation and to the quality of any reprocessed rubber.

As current methods of disposal of waste rubber are environmentallyunacceptable and conventional methods of recycling produce low qualityrubber materials, there remains a need for a method of recycling rubbermaterials which produces rubber of high quality and reducesenvironmentally unacceptable consequences.

SUMMARY OF THE INVENTION

The present inventors have surprisingly discovered that hydrophobicbacteria for example mycolic acid containing actinomycete bacteria or‘mycolata’ can efficiently devulcanise waste rubber products withoutcausing significant degradation of the unsaturated hydrocarbon polymerforming the rubber chains. The rubber recycled using these bacteria maybe revulcanised to produce rubber material of a much higher quality thanconventional recycled rubber materials.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the chemical structures of (a) natural rubber(cis-1,4-polyisoprene, (b) synthetic styrene butadiene rubber (SBR), and(c) the model compound allyl disulphide.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, in a first aspect of the present invention, there isprovided a method of devulcanising rubber comprising, providing avulcanised rubber substrate, exposing the vulcanised rubber substrate tomycolata bacteria, and allowing the mycolata bacteria to break down C—Sand S—S bonds in the vulcanised rubber substrate to produce adevulcanised rubber.

Throughout this specification, reference to devulcanisation refers tothe breaking of C—S and S—S bonds between rubber molecules. Reference to“devulcanised rubber” refers to rubber in which C—S and S—S bonds havebeen broken.

Preferably, devulcanised rubber produced according to the method of theinvention has less than 90%, preferably less than 85%, more preferablyless than 80%, even more preferably less than 75%, yet more preferablyless than 70%, even more preferably less than 65%, most preferably lessthan 60% of the C—S and S—S bonds originally present in the “vulcanised”form prior to treatment using the method of the invention. This may beassessed using conventional means of measuring total sulphur content.Preferably, the method of the invention removes at least some sulphurfrom the rubber substrate. Thus, preferably, devulcanised rubberproduced according to the method of the invention has less than 90%,preferably less than 85%, more preferably less than 80%, even morepreferably less than 75%, yet more preferably less than 70%, even morepreferably less than 65%, most preferably less than 60% of the totalsulphur content of the “vulcanised” form prior to treatment using themethod of the invention.

Preferably, the devulcanised rubber product produced using the method ofthe first aspect of the invention is of sufficient quality to be capableof reprocessing and revulcanisation and reprocessing to a new rubberproduct e.g. rubber tyre, without the need for addition of virginrubber. However, the recycled rubber is capable of being blended andreprocessed with virgin rubbers in all proportions. Preferably, a tyreproduced using such a recycled rubber would meet or exceed the relevantsafety and quality standard set for vehicle tyres by UK, EU or USregulatory authorities.

According to a second aspect of the present invention, there is provideda method of recycling a vulcanised rubber comprising devulcanising therubber according to the first aspect of the invention, and reprocessingthe devulcanised rubber. Reprocessing may include blending with virginrubber and/or may include addition of any further ingredients prior torevulcanisation to yield a high quality rubber product. The reprocessingtemperature will generally be sufficient to kill and/or destroy most,preferably all, of the bacteria. Alternatively the devulcanisingbacteria can be removed from the rubber by alkali washing prior tofurther processing.

Examples of mycolata bacteria which may be used in the present inventioninclude, but are not limited to, members of the genera Corynebacterium,Rhodococcus, Nocardia, Gordonia, Tsukamurella, Dietzia andMycobacterium. In a preferred embodiment, the bacterium is of the genusGordonia. In a particularly preferred embodiment, the bacteria is theGordonia desulfuricans strain SG213E, samples of which were depositedwith the National Collections of Industrial and Marine Bacteria Ltd.(NCIMB), 23 St Machar Drive, Aberdeen, Scotland AB24 3RY on 28 Feb. 2003and also on 29 Jul. 1996 under accession no NCIMB 40816.

The methods of the invention may be used on any type of vulcanisedrubber, in particular waste rubber, such as rubbers used in vehicletyres. Rubbers for use in the invention may be natural or synthetic or amixture thereof. Synthetic rubbers include but are not limited toStyrene Butadiene Rubbers (SBR) and Polybutadiene Rubbers (BR). Examplesof structural formulae for vulcanised natural rubber, synthetic StyreneButadiene rubber and the model compound allyl disulphide are shown inFIG. 1.

In order to maximise the surface area of rubber material for reactionwith the bacteria, the rubber material is preferably provided inparticulate form e.g. such as in the form of rubber crumb, thepreparation of which is known to the person skilled in the art.Preferred particle, e.g. crumb, sizes are in the range 0.1 mm to 15 mm,for example 1 mm to 15 mm. Such particles, e.g. crumbs, may be formedusing any method known in the art, for example mechanical and/orcryogenic grinding.

Moreover, the present inventors have found that the efficiency of themethods of the invention and the quality of the devulcanised rubberproduced using the methods of the invention may be enhanced by carryingout the reaction in the presence of rubber processing oils such ashydrocarbon oils, and similar materials which contain long hydrocarbonsequences in the molecular structure e.g. stearic acid. In a preferredembodiment the rubber processing oil is stearic acid. In a furtherpreferred embodiment, the rubber processing oil is hexadecane.

Without being limited to a particular mechanism it is believed that bycoating the rubber particles with such oils, or similar materials, thesurface area on which the bacteria may act is increased and access tothe C—S and S—S bonds is improved, enabling more efficient breaking ofthe C—S and S—S bonds and preferably removal of sulphur to leave adevulcanised rubber. Moreover, by coating the rubber particles in suchoil, or similar materials, the bacteria may use the oil as a Carbonsource for growth instead of using the rubber hydrocarbon polymer, thusfurther limiting the degradation of the rubber hydrocarbon chains.Accordingly, in particularly preferred embodiments of the invention, therubber material e.g. in the form of rubber crumb, is reacted withbacteria in the presence of oils, or similar materials, which can act ascarbon sources for the devulcanising bacteria. Suitable oils include butare not limited to mineral oils such as paraffinic, naphthenic, aromaticand white oils. Particularly preferred for use in the methods of theinvention are oils which allow swelling of the rubber with greateraccess to sulphur cross-links. In addition, the metabolism of suchrubber processing oils which may be used by the bacteria in the methodsmay result in the production of surface active agents (surfactants)which further enable access of the bacteria to the sulphide bridges.

Although any suitable temperature range may be used in the methods ofthe invention, the temperature being determined principally by theoptimum temperature required for activity of the particular bacteriabeing used, in preferred embodiments the bacteria are active in breakingthe C—S bonds and S—S bonds at mesophilic temperature ranges, e.g. inthe range 15-40° C., preferably in the range 20-35° C. The use ofbacteria which devulcanise rubber products at room temperature enablesgentler treatment of the rubber products compared to conventionaltreatments to reclaim rubber, many of which require treatment at hightemperatures. The use of mesophilic bacteria enables the breakdown ofC—S and S—S bonds between the unsaturated “rubber” hydrocarbon polymerchains, which constitute the main rubber chains, without degrading thehydrocarbon polymer.

In preferred embodiments, the method is carried out under conditions ofpH in the range pH5 to pH9, preferably in the range pH6 to pH8, mostpreferably at around pH 7 i.e. in the range pH 6.5 to pH 7.5.Preferably, the method is performed under aerobic conditions.

The methods of the invention may be carried out in any suitable reactionvessel, including biopiles, preferably with means for controllingreaction conditions. Various reaction conditions and factors maybemodified in order to control the rate and extent of bacterialdevulcanisation according to the method of the invention. For example,the rate and/or extent of bacterial devulcanisation of rubber productmay be controlled by controlling one or more of oxygen tension, redoxpotential, temperature, process oil concentration, mixing speed duringdevulcanisation, and/or physical and/or chemical treatment of the rubbersubstrate e.g. rubber crumb prior to devulcanisation according to themethod of the invention.

The rubber material may thus be pre-treated prior to reaction with thebacteria. Such pre-treatments may include method steps to at leastpartially remove textile fibres, metal beads and other constituents inthe vulcanised rubber as well as chemical and/or mechanical treatments.

Preferably, during reaction, the reaction mixture of rubber and bacteriaare agitated together, e.g. using a rotating drum. Mixing speed may beadjusted to provide control of the rate of reaction.

In preferred embodiments of the invention, the method is carried outunder conditions of undetectable or very low sulphur concentration e.g.less than 0.025%, preferably less than 0.01% sulphur such that thebacteria degrade the sulphur bonds between the rubber molecules toutilise the sulphur in the vulcanised rubber causing devulcanisation.Preferably the method is carried out whereby all trace elementsnecessary for the growth of the bacteria are present except for sulphur,

The incubation time of bacteria with the rubber substrate according tothe method of the invention may be adjusted dependent on a number offactors including particular bacteria used, temperature of reaction,size of rubber substrate particles used, presence and nature of carbonsupport. Typically, the rubber substrate will be incubated with thebacteria for an incubation time in the range 1 to 96 hours or longer.

In preferred embodiments, the oxygen tension may be in the range 0.5-20mg/L, for example 2-20 mg/L. More preferably, the oxygen tension is inthe range 0.5-9.0 mg/L. Most preferably the oxygen tension is 4-8 mg/L.The oxygen tension may however be higher depending on the systeminternal pressure.

Following treatment, the devulcanised rubber may be washed and filteredin order to remove any residual bacteria. Additionally or alternatively,any live residual bacteria may be killed by reprocessing the rubber athigh temperatures. For example, reprocessing of the devulcanised rubberwill typically take place at temperatures in excess of 100° C., forexample 150° C., which will kill bacteria such as the mycolata which arepreferred for use in the methods of the invention.

In a further aspect of the invention there is provided devulcanisedrubber produced according to the methods of the invention.

Devulcanised rubber of the invention and made according to the presentinvention has superior properties to recycled rubber produced accordingto conventional methods and may be revulcanised and used in theproduction of new rubber products, for example in the production oftyres. The quality of rubber produced by the methods of the inventionmay be tested using conventional rubber quality criteria known to thoseversed in the art. Such criteria include plasticity (e.g. ASTM StandardD1646), scorch (e.g. ASTM Standard D1646), minimum viscosity (e.g. ASTMStandard D1646), shore hardness (e.g. ASTM Standard D2240), modulus ofelongation at 300% and 100% (e.g. ASTM Standard D412, test method A),elongation at break (e.g. ASTM Standard D412, test method A), energy atbreak (e.g. ASTM Standard D412, test method A as set forth in units ofMPa), G′ (e.g. ASTM Standard D2221) and Tan Delta hysteresis measured inaccordance with ASTM Standard Db 2231 ).

The invention will now be exemplified with reference to the followingnon-limiting description and the accompanying FIG. 1, which showsstructural formulae of some vulcanised rubber molecules.

EXAMPLES

Using old tyres, rubber crumb is prepared and treated with process oilsuch as one or more mineral oils such as paraffinic, naphthenic,aromatic or white oil in a reaction vessel. A bacterial suspension ofGordonia desulfuricans strain SG213E (NCIMB 40816 or as deposited withNCIMB on Feb. 28, 2003) is mixed with the rubber crumb and process oilat room temperature (20-30° C.) for a period of 2-12 days, butpreferably 4-7 days. The presence of the oil swells the rubber crumbs,allowing enhanced access of the bacteria to C—S and S—S bonds, whileprotecting the rubber hydrocarbon chains from degradation by acting as aC source for the bacteria. Temperature, aeration, crumb mass to processoil ratio, and other physico-chemical factors are controlled to optimisereaction conditions.

After reaction, the devulcanised rubber is recovered from the reactionvessel, filtered and washed and tested for extent of devulcanisation andquality of rubber product using tests known in the art. Suitableproperties of the rubber product which may be tested include one or moreof tensile strength, modulus, hardness, tear resistance and solventswelling of the recovered rubber. Tests for the appearance of inorganicsulphur in the process effluent or liquid residue may also be carriedout, as well as total sulphur content of the rubber.

In addition, periodically during the reaction period, samples may beremoved from the reaction vessel and tested for extent ofdevulcanisation and product quality using one or more of such tests.

Example 1

50 g of 12 mesh rubber crumb derived from truck tyres was rotated in ahorizontal cylindrical vessel with 100 cm³ of aqueous medium containingessential minerals, between 0.1 and 3 cm³ of hexadecane and 500 μlinnoculum of a late logarithmic growth phase culture of Gordoniadesulfuricans (NCIMB 40816) grown in a similar medium supplemented withbenzothiophene as a sulphur source.

Typical aqueous medium composition in gdm⁻³

-   Na₂HPO₄ 4.33 g, KH₂PO₄ 2.65 g, NH₄Cl 2 g,-   MgCl₂.6H₂O 0.64 g, CaCl₂.2H₂O 33 mg, ZnCl₂ 2.6 mg,-   FeCl₂.4H₂O 2.6 mg, EDTA 1.25 mg, MnCl₂.4H₂O 1 mg, CuCl₂.2H₂O 0.15    mg,-   Co(NO₃)₂.6H₂O 0.125 mg,-   Na₂B₄O₇.10H₂O 0.1 mg, (NH₄)₆Mo₇O₂₄.4H₂O 0.09 mg.

The medium was changed when maximum growth of the bacteria was observed.This change could be repeated several times. The percentage reduction oftotal sulphur content in the rubber samples after treatment variedbetween 23% and 35%.

When blended with virgin rubber stock at a loading of 40% by weight andrevulcanised, the treated crumb samples showed good tensile strengthsubstantially higher than similar vulcanised blends containing untreatedcrumb. Eg

-   Blend with 40% treated crumb, 20.9 MPa-   Blend with 40% untreated crumb, 17.1 Mpa

Example 2

The same process was carried out as in Example 1 but with hexadecane isreplaced by glycerol. In contrast to example 1 when hexadecane isreplaced by glycerol as a carbon support there is no significantreduction in the total sulphur content of the rubber.

Example 3

The same process was carried out as in Example 1 but with Gordoniadesulfuricans replaced by a Rhodococcus erythropolis strain DT10. Incontrast to example 1, the reduction in total sulphur content of therubber was substantially lower at 13%.

Example 4

The same process was carried out as in Example 1 but with Gordoniadesulfuricans replaced by the Rhodococcus erythropolis strain used inexample 3 and the hexadecane replaced by glycerol. In contrast toexample 1 there was no significant reduction in total sulphur content ofthe rubber.

Example 5

The same process was carried out as in Example 1 but with Gordoniadesulfuricans is replaced by a further Rhodococcus erythropolis strain.in contrast to example 1 when Gordonia desulfuricans was replaced by afurther Rhodococcus erythropolis strain DT05, the reduction in totalsulphur content of the rubber was 11%.

Example 6

The same process was carried out as in Example 1 but with Gordoniadesulfuricans replaced by the same strain of Rhodoccocus erythropolis asused in example 5 and hexadecane replaced by glycerol. In contrast toexample 1 the reduction in total sulphur content of the rubber was only9%.

All documents referred to in this specification are herein incorporatedby reference. Various modifications and variations to the describedembodiments of the inventions will be apparent to those skilled in theart without departing from the scope and spirit of the invention.Although the invention has been described in connection with specificpreferred embodiments, it should be understood that the invention asclaimed should not be unduly limited to such specific embodiments.Indeed, various modifications of the described modes of carrying out theinvention which are obvious to those skilled in the art are intended tobe covered by the present invention.

1. A method of devulcanising rubber comprising providing a vulcanisedrubber substrate, exposing the vulcanised rubber substrate to mycolatabacteria, and allowing the mycolata bacteria to break down C—S and S—Sbonds in the vulcanised rubber substrate to produce a devulcanisedrubber, wherein the mycolata bacteria comprise a bacteria Gordoniadesulfuricans.
 2. The method according to claim 1, wherein thedevulcanised rubber has less than 95% of the C—S and S—S bondsoriginally present in the vulcanised rubber substrate.
 3. The methodaccording to claim 2, wherein the devulcanised has less than 70% of theC—S and S—S bonds present in the vulcanised rubber substrate.
 4. Themethod according to claim 1, wherein the devulcanised rubber has lessthan 90% of the total sulphur content of the vulcanised rubbersubstrate.
 5. The method according to claim 1, wherein the Gordoniadesulfuricans bacteria comprise Gordonia desulfuricans strain SG213E(accession no: NCIMB 40816).
 6. The method according to claim 1, whereinthe rubber substrate is in particulate from, wherein the particles havea cross section in the range 0.1 mm to 15 mm.
 7. The method according toclaim 1, wherein the rubber substrate is exposed to the mycolatabacteria in the presence of one or more processing oils.
 8. The methodaccording to claim 7, wherein the processing oil is stearic acid orhexadecane.
 9. The method according to claim 1, wherein the method isperformed at a temperature in the range 15-40° C.
 10. The methodaccording to claim 1, wherein the method is performed under conditionsof less than 0.025% sulphur.
 11. The method according to claim 1,wherein the method is performed under conditions of oxygen tension inthe range 0.5-9.0 mg/L.
 12. A method of recycling vulcanised rubbercomprising devulcanising vulcanised rubber according to thedevulcanising method of claim 1 and revulcanising the rubber product toproduce a recycled rubber product.
 13. The method according to claim 12,wherein the devulcanised rubber is reprocessed at a temperature ofgreater than 100° C.
 14. The method according to claim 12 comprising thestep of forming a tire using the recycled rubber product.
 15. Adevulcanised rubber produced according to the method of claim
 1. 16. Arecycled rubber product produced according to the method of claim 12.17. A tire comprising recycled rubber product produced according to themethod of claim
 12. 18. A tire produced according to the method of claim14.